2i8 Experimental Zoology 



based on these results. He lays much emphasis on what he 

 calls physiological units or unit-characters, but disclaims any 

 desire to locate these in any special part of the cell, and beheves 

 the evidence justifies him in supposing that each new step or 

 mutation involves a change in the organization of the germ-cell of 

 such a kind that a new unit-character appears. The change 

 may be slight or great compared with the parent form, but its 

 unity is shown by its behavior in heredity. New elementary 

 species are characterized by having at least one new physiological 

 unit. By. way of example I may cite the following cases of 

 mutations amongst animals. The appearance of the merino 

 ram was probably due to some physiological change in a germ- 

 cell of one of its parents of such a sort that one character espe- 

 cially, viz. the wool, was changed. Other less striking characters 

 were also present, and inasmuch as these are always associated 

 with the merino type of hair, they all belong together. The 

 ancon ram showed differences in the proportion of nearly the 

 whole body ; but the entire change must probably be referred 

 to a single, although profound, unit change in the germ- 

 cells. The occasional appearance of the turnspit type of dog, 

 which resembles the ancon ram in the form of its body, indicates, 

 if it does not prove, that this kind of change, involving nearly 

 all the parts of the body, may readily occur. So numerous are 

 the parts affected in these cases that it is impossible to ascribe 

 the results to "accidental" combinations of the different organ 

 elements, for it is inconceivable that just these combinations 

 could ever appear more than once, but must be due to some 

 definite change in the germ-cells which can appear in this 

 particular way. The same statement must also hold for 

 Lamarck's primrose, where, however, instead of one possible 

 combination there are several possible ones that become reahzed. 

 If we use Galton's oft-quoted comparison of a polyhedron, we can 

 see that resting on its most stable face it may be rocked back and 

 forth, but always returns to its same resting-place. Such oscil- 

 lations on a single face would correspond to the fluctuating varia- 

 tions of a species. Should a greater movement take place, the 



